Heretofore, lead-acid batteries, which are relatively low-priced and well-proven in use, have been widely used as power supplies for electric automobiles, for example. Meanwhile, lithium-ion batteries, which are high-voltage-available and include high-energy density, have also been used as the power supplies in recent years.
However, the charge receiving performance of lead-acid batteries is low, so that it is difficult to efficiently store regenerative energy which is available during braking of vehicles such as electric automobiles (hereinafter, simply referred to as “vehicle” or “vehicles”). Meanwhile, lithium-ion batteries involve high manufacturing costs, and it is difficult to increase the capacity of lithium-ion batteries. For this reason, sufficiently ensuring the power required to start the engine of a vehicle, using lithium-ion batteries is difficult.
In this respect, Patent Literature (hereinafter, referred to as “PTL”) 1 and PTL 2, for example, disclose a technique that connects in parallel a lithium-ion battery having a low internal resistance and a lead-acid battery having a large capacity. According to the technique, a battery system having a high charge receiving performance and a large capacity can be obtained, inexpensively.